The present invention relates generally to aeronautical vehicle fuel tank inerting systems, and more particularly, to a system for inerting fuel tank(s) of a commercial aircraft.
Fuel tanks within wings of an aircraft represent vulnerable areas on all aircraft for the potential for flame initiation due to the existence of fuel vapor and oxygen concentration levels therein. Fuel tank inerting systems are currently used to reduce oxygen concentration levels within fuel tanks of some military aircraft in order to significantly reduce the vulnerability of military aircraft to hostile munitions.
As is known in the art, as oxygen concentration levels within a fuel tank increase, likelihood of flame initiation and propagation and likelihood of a possible explosion also increases. This threat exists since fuel vapors generally mix with ambient air that has a 21% concentration of oxygen. It has been determined that when oxygen concentration levels are maintained at a level of approximately 12% or less at seal level (14.5% at 40,000 ft), that the threat of flame propagation or of an explosion from occurring can be significantly reduced or eliminated.
Some vehicles, principally military vehicles, are equipped with fuel tank inerting systems, which supply nitrogen gas to purge fuel tanks and effectively reduce oxygen concentration levels therein. Of available types of fuel tank inerting systems, the most desirable from a weight, capacity, and ground service requirements stand point is an inert gas generation system that utilizes pressurized air supplied by engine bleed from a gas turbine engine or other airborne source of pressurized air. This air is separated into an oxygen rich component, which is exhausted overboard, and an oxygen depleted or inert gas component, which is fed to the fuel tank.
Unlike military applications, commercial aircraft ignition threat requirements are not as probable, but other requirements, such as reliability, maintenance and cost of internal components and systems can be more stringent. Military inerting systems often utilize complicated and unreliable components to provide nitrogen-enriched air to each wing fuel tank, for all perceivable mission conditions. During combat military missions, threats from hostile munitions are highly probable. The military type systems have a poor reliability history, with high maintenance costs and are oversized for the vast majority of typical commercial aircraft operations.
The primary flammability exposure for current commercial aircraft is in the center fuel tanks, particularly if located adjacent to heat sources. Thus, a primary desire exists in commercial aircraft applications to reduce flammability exposure in center fuel tanks, to a level that is similar to that of the wing fuel tanks. Additionally, reducing exposure in wing fuel tanks can also be desirable when aircraft design characteristics result in high flammability exposure or when additional reduction in wing fuel tank flammability exposure is desired.
Additionally, it is also desirable for the fuel tank to be inert during both ground and flight conditions. The inerting air need be supplied without use of inerting air storage tanks, which can be heavy in weight, as are commonly used in prior art military inerting systems. Further, variations in oxygen concentrations throughout tanks must be minimized to achieve a uniform level of inert content, without over sizing the inerting system.
It is therefore desirable to provide an inerting system that reduces flammability exposure in fuel tanks of a commercial aircraft during both ground and flight conditions, while at the same time minimizing size, weight, maintenance, and cost, as well as maximizing reliability to be effective and feasible for commercial applications.